bin/helpers: use proper channel when installing LXD from custom snap (#…

…401)

Before, doing `snap install lxd` would pull LXD `5.21/stable` which is
ATM based on `core22` but chances are that the custom LXD snap provided
is a modern one based on `core24`. As such, you end up pulling `core22`
just for getting your custom snap installable.

Instead, pull the official LXD snap from the requested channel which
still works to give us the assert required to then be able to install
custom LXD snap. This way, a virgin VM won't need to uselessly download
`core22`.

bin/helpers

@@ -252,7 +252,7 @@ install_lxd() (
         # snapstore then install with --dangerous. The alias needs to be created
         # manually too.
         if [ -n "${LXD_SNAP_PATH:-}" ]; then
-            snap list lxd 2>/dev/null || snap install lxd
+            snap list lxd 2>/dev/null || snap install lxd --channel "${LXD_SNAP_CHANNEL}" --cohort=+
             snap install --dangerous "${LXD_SNAP_PATH}"
             snap alias lxd.lxc lxc
         else